extern variables

**karnavor** · 03-29-2010

Hello,

Suppose I have a class, MyClass, which contains variables (int x, y, z) which I want to access throughout my program. I want to be able to use the class in a static sense. I want to be able to access the variables using something like "MyClass::x", without actually having to create a MyClass object. So, I need to initialize these variables somewhere, which I gather requires the use of "extern".

Here is what I have so far:

Code:

// myclass.cpp
class MyClass
{
    int x;
    int y;
    int z;
};

// main.cpp
int main()
{
    extern MyClass::x = 4;
    extern MyClass::y = 5;
    extern MyClass::z = 8;

    return 0;
}

But this is giving me a load of error messages, so the approach is obviously wrong. How should I go about it?

Thanks

**laserlight** · 03-29-2010

Originally Posted by karnavor

I want to be able to access the variables using something like "MyClass::x", without actually having to create a MyClass object.

You can define the class as:

Code:

class MyClass
{
public:
    static int x;
    static int y;
    static int z;
};

Then in exactly one source file, write:

Code:

int MyClass::x = 4;
int MyClass::y = 5;
int MyClass::z = 8;

This will be effectively using the class purely as a namespace.

Are you really sure that you need such global variables in the first place? Or do you want global constants instead?

**karnavor** · 03-29-2010

Well yes, actually I want them to be constants as they will not change. Can I do it in the same way, but using "const static int x" instead of just "static int x"?

**MK27** · 03-29-2010

Wouldn't they be better as defines then?:

Code:

#define MYX 4
#define MYY 5
#define MYZ 6

**laserlight** · 03-29-2010

In that case, since they are all integral class constants, this will suffice:

Code:

class MyClass
{
public:
    static const int x = 4;
    static const int y = 5;
    static const int z = 8;
};

**karnavor** · 03-29-2010

It seems to work well when I initialize the values in a source file:

Code:

int MyClass::x = 4;
int MyClass::y = 5;
int MyClass::z = 8;

But I thought that it was bad practice to have global operations such as this. So, I tried to create an initializer function which would perform this step inside a class:

Code:

void InitializeVariables()
{
int MyClass::x = 4;
int MyClass::y = 5;
int MyClass::z = 8;
};

But then I get the following error: "definition or redeclaration illegal in current scope". Does this mean that these values always have to be defined in global scope?

**cpjust** · 03-29-2010

Originally Posted by MK27

Wouldn't they be better as defines then?:

Code:

#define MYX 4
#define MYY 5
#define MYZ 6

No, const is the way to go. I only use #define if I have no other choice.

**laserlight** · 03-29-2010

Originally Posted by karnavor

But I thought that it was bad practice to have global operations such as this.

The bad practice is to have global variables in the first place. But since they are global constants, it is quite okay, especially since you are just initialising them to literal values (i.e., there can be no static initialisation order problem where you initialise them with other namespace-level objects, from another translation unit, that might not have been properly initialised at that point).

Originally Posted by karnavor

So, I tried to create an initializer function which would perform this step inside a class:

You cannot initialise them in that way; you can only assign to them, but obviously you also cannot do that since you want them to be const.

**Elysia** · 03-29-2010

Another option is a namespace...

Code:

namespace MyNameSpace
{
    const int x = 0;
    const int y = 1;
    const int z = 2;
    // etc
}

It looks better than abusing a class for global constants that usually has little do with the class itself.

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